1. Field of the Invention
The present invention relates to an optical radar apparatus for a vehicle identifying an obstacle to the vehicle by detecting pulsed laser beams reflected from objects.
2. Description of the Related Art
Follow-up running controllers disclosed in, for example, Japanese Patent Laid-Open Nos. 55-86000 and 60-239900, are known as conventional vehicular optical radar apparatuses. Each of the controllers monitors objects lying ahead of a vehicle by means of a radar apparatus, which uses light or radio waves, provided at the forward portion of the vehicle so as to detect an obstacle such as a preceding vehicle running ahead of the vehicle (hereinafter referred to as a subject vehicle) on which the radar apparatus is mounted. Thus, such controllers are used to control the speed of the subject vehicle so that a safe distance between the subject vehicle and a preceding vehicle can be maintained.
Moreover, an optical radar apparatus disclosed in Japanese Patent Publication No. 3-30117 is known as another type of conventional vehicular optical radar apparatus. Such an apparatus outputs a received-light signal when the set intensity of the reflected light is reached. Then, various values representing the distance to the object are obtained according to a plurality of the received-light signals which are output within the scanning angle. When such a disparity of the distance is equal to or less than a predetermined value, the optical radar apparatus determines and outputs the value representing the distance detected as a distance which should be kept between the subject vehicle and a preceding vehicle. The apparatus determines the set intensity of the reflected light according to the reflectance factor of reflectors, thereby enabling the detection of the reflectors only with high accuracy. The apparatus also detects the distance to the reflectors for a plurality of times within the set scanning angle, that is, within the width of a preceding vehicle. Then, when the values representing the distance are substantially equal to each other, the apparatus determines that the detected obstacle is a pair of reflectors in a pair of tail lamps provided on a preceding vehicle and outputs the detected distance as a distance which should be kept between the subject vehicle and the preceding vehicle.
However, the above-mentioned follow-up running controllers used as conventional vehicular optical radar apparatuses pose the following problems. Since they are unable to identify the type of the detected obstacle, they can not discern or predict road environments such as the kind of a road (non-expressway, expressway) on which the subject vehicle is running, and they also can not sense or predict a running environment of a lane such as a curved or straight course in which the subject vehicle is running. Thus, when identifying a preceding vehicle, the apparatus is sometimes adversely influenced by the road environment and the running environment. Accordingly, there often is a case in which the apparatus erroneously identifies a preceding vehicle, that is, it fails to identify with high precision a preceding vehicle running in the same lane as the subject vehicle. For example, at a curve, such an apparatus as a conventional follow-up running controller sometimes mistakes a vehicle running in the adjacent lane for a preceding vehicle in the same lane as the subject vehicle, and also erroneously identifies a road sign or a delineator disposed on the surface of a road as a preceding vehicle. Such errors caused by the radar apparatus used as a follow-up running controller not only impair riding comfort, but also may lead to a serious accident.
The above problems can be solved by such means as identifying a delineator. Then, the transverse position of the subject vehicle from the delineator can be calculated by the detected distance and angle, thereby assuming an environment of the lane in which the subject vehicle is running. Besides, the identification of a delineator makes it possible to infer the curvature of a road, thereby assuming an environment of the lane in which the subject vehicle is running. Thus, it is possible to identify a preceding vehicle running in the same lane as with the subject vehicle.
However, the conventional optical radar apparatus described above also presents the following problems. Namely, since it identifies a preceding vehicle merely by detecting a pair of reflectors provided on a rear end thereof in a transversely spaced relation, it is unable to recognize a road environment, a running environment, and the like. Hence, it is impossible for the conventional optical radar apparatus to determine whether the preceding vehicle detected is running in the same lane as with the subject vehicle, thus resulting in failing to accurately identify a preceding vehicle.